(1) Field of the Invention
The present invention relates generally to magnetic bubble domain devices. More specifically, the preferred embodiment of the present invention is related to a redundant bubble domain detector for sensing the presence or absence of a magnetic bubble domain in a magnetic bubble domain memory device.
(2) Description of the Prior Art
Magnetic bubble domain memory devices are conventionally made by sandwiching a thin film of magnetic material between two permanently biased magnets. Specifically, epitaxial garnet film is often used as the magnetic material, and the external bias field provided by the magnets is typically 180 Oe. Logical "ones" are represented by the presence of domains of reverse magnetization in this film and logical "zeros" are represented by the absence of the reverse domains. The reverse domains in the epitaxial garnet film, known as bubble domains, are stabilized by the presence of the external bias field created by the two permanent bias magnets. These bubble domains are typically cylindrical in shape, aligned with the external bias field, and approximately 1-5 microns in diameter.
The bubble domains produce a magnetic field above the garnet film surface similar to that of a magnetic dipole. It is by means of this field that the bubble domains can be moved in the plane of the film. Typically, the bubble domains are moved by a method known as field-access propagation. In this method, the garnet film in which the bubble domains occur has thin-film permalloy patterns deposited on it. These patterns lie in the plane of a rotating magnetic field generated by two coils. The changing poles periodically induced in the permalloy patterns by the rotating magnetic field alternately attract and repel the bubbles, causing them to propagate along preselected paths. An asymmetric chevron propagation element is currently regarded as the most desirable permalloy pattern.
In most existing and proposed bubble memory chips, the bubble memory architecture comprises minor loops which operate as shift registers. The bubbles domains recirculate in the minor loops and are generated and detected in input and output tracts. These tracts and the minor loops are organized in different ways to a large extent dictated by the other components. Specifically, present bubble memory architectures are largely a function of the design of the bubble generators, replicators, detectors, and the gates used for transferring or swapping bubble domains in and out of the minor loops.
To detect bubble domains and generate the desired electrical response, an array of permalloy chevron propagation patterns stretches the bubble domain to several hundred times its original diameter. These stretched bubbles, known as stripes, then pass under a magneto-resistive sensing element also made of permalloy. The presence of a stripe changes the magnetic field strength through the permalloy, thereby altering the permalloy's resistance. This change in resistance is detected electronically and provides an indication of the presence or absence of a bubble domain.
Most present bubble memories have a surplus of minor loops to provide a built-in redundancy to insure the high probability of a working memory device in spite of defects in the magnetic material or in the permalloy patterns which cause one or more of the minor loops to be inoperative. However, the detectors and the associated propagation and stretching arrays require a significant fraction of the total chip area as a result of the requirement to stretch the bubbles to a length which is sufficient to be passively detected by magneto-resistive detectors. On memory devices where two or more detectors must function on a single die, the potential of a detector failure creates a high loss in yield of properly functioning devices.